Make gollum use a branch instead of master

The idea is to have something similar to gh-pages, were the wiki edition will not affect the development edition.

There is a new branch, called wiki, where the wiki pages are stored.


The idea to make this a pull request is to discuss the approach, making it more visible to everybody.
Me and @cv discussed a little bit about and I would like to hear some feedbacks on it.

It still is required to make a job to update the repo with the editions on the wiki pages.
We can keep this pull request open while we are working on it.

Please do a `git pull --rebase && git checkout -b wiki origin/wiki` to update your git repo with the wiki data.